Method of forming a hermetically sealed fiber to chip connection

ABSTRACT

Disclosed are methods of providing a hermetically sealed optical connection between an optical fiber and an optical element of a chip and a photonic-integrated chip manufactured using such methods.

FIELD OF THE INVENTION

Embodiments of the invention relate to optical interconnects forchip-to-chip and intra-chip communication, and specifically directed tomethods of forming a hermetically sealed connection between an opticalfiber and a silicon based photonic-integrated-chip.

BACKGROUND OF THE INVENTION

Optical transmission may be used as a means for communication betweenseparate integrated circuit chips (inter-chip connections) and withincomponents on the same chip (intra-chip connections). In chip-to-chipcommunication via optical interconnects, each chip on the circuit boardis interfaced with a transmitter-receiver optoelectronic chip, and thetwo optoelectronic chips are connected via a planar dielectric waveguideor optic fiber. Likewise, optical waveguides may be used to connectcomponents within a chip, such as between an integrated optical sourceand a detector. An integrated optical waveguide is an optical pathformed on or within a dielectric substrate, e.g., oxide coated siliconsubstrate, using lithographic processing. The waveguide can be made ofan inorganic crystal or semiconductor material having a higher index ofrefraction than the chip substrate to guide optical signals along thewaveguide.

The coupling of a single-mode fiber to an integrated optical waveguide(and vice versa) is one of the most expensive and time-consumingmanufacturing processes in the packaging of semiconductor photonics.Various solutions to the coupling problem have been proposed includingusing a lateral inverted taper structure or a vertical diffractivegrating structure.

Another challenge is to hermetically seal the fibers or wires connectedto the photonic-integrated chip because the performance of photonicelements may be adversely affected by environmental conditions such asmoisture and contaminants. Therefore, environmental isolation of thephotonic elements in the chip is a design challenge. FIG. 1A shows aconventional photonic integrated chip package 100 connected to anoptical fiber 110 through a feedthrough 120. Feedthrough 120 provides ahermetic seal between the fiber 110 and the chip package 100. FIG. 1Bshows a cross-section diagram of the hermetic fiber feedthrough 120. Thefeedthrough 120 encases an end stripped portion 130 of the optical fiber110. The end stripped portion 130 of the optical fiber 110 is surroundedby a glass solder 140 material, such as lead borate glass. The glasssolder 140 is stacked between a glass sleeve 150 and the fiber 110thereby forming a bond between the fiber 110 and the glass sleeve 150that is largely free from porosity. The glass sleeve 150 is encased by aglass solder 160 material, such as, lead borate glass, which in turn issurrounded by a outer sleeve 170. The outer sleeve 170 is made frommetal, metallic alloy, ceramic, or glass. The end face 180 of thehermetically sealed fiber 110 is coupled to the photonic integrated chippackage 100.

The conventional method described above however is costly and does notsupport high volume manufacturing. There is a need for an improvedmethod to hermetically seal an optical fiber to a photonic-integratedchip. The method needs to be low cost and provide for a hermeticallysealed connection with high reliability under extreme ambientconditions. In addition, the method needs to support high volumemanufacturing processes and low processing temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a conventional hermetically sealed fiber to chipconnection;

FIG. 2 is a top view of an optical device in accordance with a disclosedembodiment;

FIGS. 3A and 3B are top and cross-sectional views, respectively, of anoptical fiber in accordance with a disclosed embodiment;

FIG. 3C is another cross-sectional view of an optical fiber inaccordance with a disclosed embodiment;

FIGS. 4A and 4B are top and cross-sectional views, respectively, of aphotonic-integrated chip in accordance with a disclosed embodiment; and

FIG. 5 is a flowchart of an integrated optical device manufacturingprocess in accordance with a disclosed embodiment.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments that may be practiced. Itshould be understood that like reference numbers represent like elementsthroughout the drawings. These embodiments are described in sufficientdetail to enable those skilled in the art to make and use them, and itis to be understood that structural, material, electrical, andprocedural changes may be made to the specific embodiments disclosed,only some of which are discussed in detail below.

Described herein is a method of preparing an optical fiber for couplingwith, for example, a photonic-integrated chip. The method includesactivating an end surface of the optical fiber in a plasma to generatedangling bonds on the end surface of the optical fiber, whereby thedangling bonds facilitate coupling with a surface of thephotonic-integrated chip. Also described is an optical devicemanufacturing process that includes the steps of pre-activating an endsurface of at least one optical fiber in a plasma generated from a gasspecie containing oxygen, nitrogen, argon, hydrogen or ammonia andconnecting the pre-activated end surface of the at least one opticalfiber to, for example, a photonic-integrated chip. The disclosed methodscan be used to manufacture an integrated optical device having ahermetically sealed fiber to chip connection.

FIG. 2 is a top view of an optical device 260 in accordance with adisclosed embodiment. The optical device 260 has a photonic-integratedchip 200 coupled to an optical fiber assembly 300. The optical device260 integrates multiple photonic functions on the photonic-integratedchip 200 using photonic elements 270 such as waveguides, opticalamplifiers, modulators, filters, optical sources and detectors. The chip200 has optical waveguides 230 that can be used to connect multiplephotonic elements 270, such as between an integrated optical source anda detector, to each other. The optical waveguide 230 can also be used toconnect a photonic element 270 to an optical fiber assembly 300 as shownin FIG. 2.

The optical device 260 has a hermetically sealed fiber to chipconnection 250 located on a side surface 240 of the chip 200. Aself-aligned mechanical interface 280 is used to help guide the fiber300 to chip connection 250 using inverted taper coupling, although otherknown coupling mechanism can be utilized. As shown FIG. 2, for example,the self-aligned mechanical interface 280 located on the side surface240 of the chip 200 is chamfered at 420 a and 420 b to mechanicallyalign with a chamfered end surface 310 of the fiber assembly 300.Although FIG. 2 shows the fiber to chip connection 250 having achamfered interface, it shall be appreciated that the fiber to chipconnection 250 can be based on any suitable shape and configuration aslong as the optical fiber assembly 300 can be aligned with and couple tothe optical waveguide 230 on the chip 200 for optical communication.

FIG. 3A is a top view of the optical fiber assembly 300. The opticalfiber assembly 300 can be a single-mode fiber having a core 220surrounded by a cladding 210 material having a lower index of refractionthan the core 220. In this embodiment, the core 220 is made of silicaand germania and the cladding 210 is made of silicon dioxide. FIG. 3Bshows a cross-sectional A-A′ view of the optical fiber assembly 300. Oneend of the optical fiber assembly 300 has chamfered surfaces 320 a, 320b. The chamfered surfaces 320 a, 320 b can have any angle a, forexample, 45 degrees, measured with respect to the center axis 330 of thefiber core 220 as shown in FIG. 3A. The chamfered surfaces 320 a, 320 bcan be annular as shown in FIG. 3C. Typically, any buffer and coating isstripped off of the end surface 310 and the chamfered surfaces 320 a,320 b prior to connecting with the chip 200.

To ensure a good seal and adhesion between the fiber assembly 300 andthe chip 200, the end surface 310 of the fiber assembly 300 ispre-activated in a plasma generated from oxygen or hydrogen containinggas species. Dangling bonds are generated on the end surface 310 offiber assembly 300 when the silicon dioxide cladding 210 (FIG. 2) andsilicon core 220 (FIG. 2) are activated in the plasma. The silicon atomrequires four bonds to fully saturate its valence shell. In crystallinesilicon, each silicon atom is bonded to four other silicon atoms. At thesurface of the silicon core 220, however, the silicon atoms may have toofew bonds to satisfy its valence shell. The surface 220 silicon atomsmay be bonded to only three silicon atoms, leaving one unsatisfiedvalence bond, also known as a dangling bond. The surface of the silicondioxide cladding 210 also has silicon atoms with unsatisfied valencebonds, or dangling bonds. In order to gain enough electrons to filltheir valence, the silicon atoms with dangling bonds on the end surface310 favor forming covalent bonds with silicon atoms that form at theside surface 240 (FIG. 2) of the optical waveguide 230 of thephotonic-integrated chip 200. Pre-activation of the silicon dioxide andsilicon end surface 310 in plasma to generate dangling bonds thusfacilitate very robust bonding between the fiber assembly 300 and thephotonic-integrated chip 200. The entire or a portion of the end surface310 of the fiber assembly 300 can be pre-activated in the plasma. Thechamfered surfaces 320 a, 320 b can also be pre-activated in the plasmaused to pre-activate the end surface 310 of the fiber assembly 300.

Other suitable gas species containing, for example, nitrogen, argon andammonia, can be used to generate the plasma. The plasma can be generatedusing any suitable process including, but not limited to, reactive ionetching plasma and microwave radicals generated from the gas species.Surface activation of the end surface 310 of the fiber assembly 300prior to bonding the end surface 310 with the photonic-integrated chip200 has the advantage that no intermediate layer, such as an adhesive,or step is needed to create a good seal and adhesion between the fiberassembly 300 and the chip 200.

FIG. 4A is a top view of the photonic-integrated chip 200. FIG. 4B showsa cross-sectional B-B′ view of the photonic-integrated chip 200. Thechip 200 includes an optical waveguide 230 formed on a dielectricsubstrate, e.g., oxide coated silicon substrate 400. The waveguide 230connects a photonic element 270 (e.g., optical source, detector, etc.)to another photonic element or is aligned with an optical fiber 300 toguide optical signals from the input optical fiber 300 to the photonicelements on the chip. One side 240 of the chip 200 has chamferedsurfaces 420 a, 420 b. The chamfered surfaces 420 a, 420 b have anglescorresponding to the angle a of the chamfered surfaces 320 a, 320 b(FIG. 3A) on the fiber assembly 300 such that the pre-activated endsurface 310 of the fiber assembly 300 aligns and couples to the surface410 connecting the two chamfered surfaces 420 a, 420 b of the chip 200.The chamfered surfaces 420 a, 420 b have a height d measured from thesurface 410 to the side surface 240 of the chip as shown in FIG. 4A.

FIG. 4B shows the silicon substrate 400 extends below the surface 410where the fiber 300 couples to the chip 200. However, it shall beappreciated that the fiber 300 may extend below the silicon substrate400 in the case, for example, where the silicon substrate 400 is acomposite structure having a thickness h of about 50 μm and the diameterof the single mode fiber, such as SMF-28, is 125 μm.

The pre-activated end surface 310 of the fiber 300 forms a hermeticallysealed connection 250 after any conventional method of assembling andalignment of the fiber 300 to the chip 200. Although FIGS. 4A and 4Bshow the optical fiber assembly 300 is aligned to the chip usinginverted taper coupling, it shall be appreciated that the embodiment maybe modified to use vertical taper coupling or any other suitablecoupling of the fiber to chip. The hermetically sealed connection 250exhibits excellent water-repellency. Standard low temperature annealingused for chip packaging flow, for example, at a temperature of at least200° Celsius for at least two hours, can provide further improvements inhermetically sealing the fiber assembly 300 to the chip 200.

Although a good seal can be created from only pre-activating the endsurface 310 of the fiber assembly 300, the surface 410 of the chip 200can be pre-activated in plasma from a gas species containing oxygen,hydrogen, nitrogen, argon or ammonia, such as the plasma used topre-activate the end surface 310 of the fiber assembly 300. The danglingbonds of the pre-activated surface 410 of the chip 200 generate evenstronger covalent bonds with the pre-activated end surface 310 of thefiber assembly 300. The chamfered surfaces 420 a, 420 b of the chip 200can also be pre-activated using the same plasma.

FIG. 5 is a flowchart of an integrated optical device manufacturingprocess in accordance with a disclosed embodiment. The process shownhermetically seals at least one input optical fiber to a surface of aphotonic-integrated chip. At step 500 of the process, the end surface310 of the optical fiber 300 that will be coupled to the chip 200 ischamfered as shown in FIGS. 3A and 3B, for example. At step 510, thesurface of the chip 200 that will be hermetically connected to the fiber300 is chamfered as shown in FIGS. 4A and 4B, for example. At step 520,the end surface 310 of the fiber 300 is pre-activated in a plasma togenerate dangling bonds (or free silicon bonds) on the end surface 310.If multiple fibers 300 are to be coupled to the chip 200 in batchprocessing, then the end surfaces 310 of the plurality of fibers 300 maybe simultaneously pre-activated in the plasma. The pre-activated endsurfaces 310 are connected to the corresponding surfaces 410 on the chip200. The connection step should occur within a predetermined time, suchas within two hours of the pre-activating step, to ensure the fiberremains pre-activated. To speed up the bonding process, at step 530, thesurface 410 of the chip 200 may also be pre-activated in the plasma togenerate dangling bonds on the surface 410 of the chip 200. Thepre-activation of the chip surface 410 may occur before, after orsimultaneous with the pre-activation of the fiber 300. At step 540, thepre-activated fiber 300 is connected to the chip 200 as shown in FIG. 2.Pressure at, for example, 1.5 MPa may be applied to ensure no gaps formbetween the fiber 300 and the chip 200. At step 550, after the fiber 300is connected to the chip 200, the chip 200 can be annealed at atemperature of at least 200 degrees. Celsius to further augment thesealing of the fiber to the chip.

While disclosed embodiments have been described in detail, it should bereadily understood that the invention is not limited to the disclosedembodiments. Rather the disclosed embodiments can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described.

1. A method of preparing an optical fiber for coupling, comprising:activating an end surface of the optical fiber in a plasma to generatedangling bonds on the end surface of the optical fiber, whereby thedangling bonds facilitate coupling with a surface of a chip.
 2. Themethod of claim 1, wherein the plasma is generated from a gas speciecontaining oxygen, hydrogen, nitrogen, ammonia or argon.
 3. The methodof claim 2, wherein the activating step is performed using reactive ionetching plasma or microwave radicals generated from the gas specie. 4.The method of claim 1, further comprising chamfering a portion of theend surface of the optical fiber.
 5. The method of claim 4, furthercomprising activating the chamfered portion of the end surface of theoptical fiber.
 6. The method of claim 5, wherein the chamfering step isperformed before the activating the chamfered portion of the end surfacestep.
 7. An optical device manufacturing process, comprising:pre-activating an end surface of at least one optical fiber in a plasmato generate dangling bonds on the end surface of the optical fiber; andconnecting the pre-activated end surface of the at least one opticalfiber to a surface of a chip.
 8. The process of claim 7, wherein theplasma is generated from a gas specie containing oxygen, hydrogen orargon.
 9. The process of claim 7, further comprising the step ofannealing the chip at a temperature of at least 200 degrees Celsius,wherein the annealing step is performed after the connecting step. 10.The process of claim 7, further comprising chamfering a portion of theend surface of the at least one optical fiber, wherein the chamferingstep is performed before the pre-activating step.
 11. The process ofclaim 10, further comprising chamfering the surface of the chip suchthat the chamfered surface of the chip aligns with the chamfered endsurface of the at least one optical fiber.
 12. The process of claim 7,wherein the pre-activating step is performed on a plurality of opticalfibers, and the connecting step occurs within two hours of thepre-activating step.
 13. An optical device, comprising: aphotonic-integrated chip on which an optical waveguide is formed; and anoptical fiber assembly comprising an optical fiber having an end surfacethat is pre-activated in a plasma to create dangling bonds whichfacilitate coupling with the optical waveguide, the end surface of theoptical fiber providing a hermetically sealed optical connection to theoptical waveguide formed in the photonic-integrated chip.
 14. Theoptical device of claim 13, wherein the optical fiber is connected tothe optical waveguide using an inverted taper.
 15. The optical device ofclaim 13, wherein a portion of the end surface of the optical fiber ischamfered and the chamfered end surface of the optical fiber aligns witha corresponding chamfered side of the photonic-integrated chip.
 16. Theoptical device of claim 15, wherein the corresponding chamfered side ofthe photonic-integrated chip is pre-activated in the plasma.